This application claims the priority of German Patent Document No. 101 52 804.3, filed Oct. 25, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an internal combustion engine with an exhaust turbocharger and an exhaust-gas recirculation device.
DE 198 57 234 A1 discloses a pressure-charged internal combustion engine, the exhaust turbocharger of which is equipped with variable turbine geometry for the variable setting of the inlet flow cross section to the turbine rotor. Adjusting the variable turbine geometry changes the effective inlet flow cross section, thereby adjusting the exhaust backpressure in the exhaust section between the cylinder outlet of the internal combustion engine and the inlet of the turbine in a specific way and allowing the power consumption of the turbine and, as a result, also the compressor power of the compressor to be adjusted. It is thereby possible to achieve increases in power both in the powered driving mode and in the engine-braking mode.
To improve the exhaust behaviour, in particular to reduce NOx emissions, the internal combustion engine is equipped with an exhaust-gas recirculation device, by means of which exhaust gas can be recirculated from the exhaust system into the intake duct. The level of the mass flow of exhaust gas to be recirculated is set as a function of current state variables and operating parameters.
The exhaust turbine has two spiral flow passages, which are separated by a dividing wall. Each flow passage is supplied with exhaust gas via a gas line assigned to it. It is possible for the exhaust gas of a respective fraction of the cylinder outlets of the internal combustion engine to be discharged via each exhaust line. The two flow passages have flow cross sections of different sizes, the recirculation line of the exhaust-gas recirculation device branching off from the smaller flow passage, in which a higher exhaust backpressure prevails owing to the smaller cross section, promoting exhaust-gas recirculation.
The dimensioning of the exhaust turbocharger depends on the size of the internal combustion engine used and is generally determined from empirical values. However, determining the size ratio of the two flow passages in the exhaust turbocharger to one another is problematic. However, optimum dimensioning is a prerequisite for clean exhaust behaviour of the internal combustion engine.
Starting from this prior art, a problem underlying the invention is to optimize an internal combustion engine in such a way that pollutant emissions and fuel consumption are reduced. In particular, the intention is to specify a relationship for dimensioning the two flow passages of the exhaust turbocharger relative to one another.
To establish the size ratio of the two flow passages, an asymmetry factor FAsym is defined from the ratio of the turbine throughput parameters xcfx86f11,S and xcfx86f12,S, which are to be determined in the region of the choke line of the exhaust turbine from the mass flow of exhaust gas, the exhaust-gas temperature and the exhaust-gas pressure. According to the invention, the maximum value of this ratio of the turbine throughput parameters of the two flow passages, may be no greater than the reciprocal of the total displacement of the internal combustion engine, raised to the power 0.15.
This specifies a relationship between a defining variable characterizing the geometry of the flow passages and the displacement of the internal combustion engine, making it possible to determine the size ratio of the flow passages as a function of the displacement. Using an empirical or, if appropriate, an analytical relationship, it is possible to deduce geometric characteristic values of the respective flow passage from the ratio of the turbine throughput parameters. In particular, it is possible, from this relationship, to establish the spiral cross section of the spiral flow passage in the inlet flow and the radial distance between the central axis in the inlet flow of the spiral cross section and the axis of rotation of the turbine rotor. In principle, these geometric variables are sufficient for the design configuration of each flow passage. With a knowledge of the total displacement of the internal combustion engine, it is thus possible to deduce the actual dimensioning of each flow passage in the exhaust turbocharger.
In an expedient development, the recirculation line of the exhaust-gas recirculation device is connected to the smaller of the flow passages. Because of its smaller cross section, the smaller flow passage has higher pressure, which can be used for the recirculation of exhaust gas from the exhaust section to the intake duct. The higher pressure in the smaller flow passage allows recirculation of exhaust gas even at low engine speeds and, as a result, improves emissions and consumption behaviour. It may be advantageous to separate the two flow passages in a pressure-tight manner from one another within the turbocharger casing in order to prevent an undesirable pressure compensation between the flow passages and to enable the pressure drop between the passages to be used in an optimum manner for exhaust-gas recirculation.
It is also advantageous to connect each flow passage to a separate exhaust line for a fraction of the cylinder outlets of the internal combustion engine, with the result that each exhaust line receives only the exhaust gas of a fraction of the cylinders of the engine. In a simple embodiment, the cylinders are divided symmetrically between the two exhaust lines. However, it may also be expedient to connect the two exhaust lines to different numbers of cylinders of the internal combustion engine. The exhaust line which carries away the exhaust gas from the larger number of cylinders is connected, in particular, to the larger flow passage.
In a preferred development, the exhaust turbine has variable turbine geometry, by means of which the inlet flow cross section between at least one flow passage and the turbine rotor can be set in a variable manner. The variable turbine geometry in the flow passage that is not involved in exhaust-gas recirculation can be opened during the powered driving mode of the internal combustion engine, in particular, in order to reduce flow resistance. As a result, the exhaust backpressure in this flow passage is reduced too, and less compression work has to be performed by the cylinders assigned to this flow passage. In the case of exhaust lines that are made separately for the two flow passages, the recirculated mass flow of exhaust gas can be adjusted independently of the position of the variable turbine geometry via the adjustment of a recirculation valve.
If appropriate, the variable turbine geometry also extends to the inlet flow cross section of the smaller flow passage, or this inlet flow cross section may be equipped with its own variable turbine geometry, which can be adjusted independently of the turbine geometry of the larger flow passage.
The relationship according to the invention for determining the size ratio of the flow passages can also be used with more than two flow passages. Thus, it is possible, in the case of a total of three flow passages, to add the turbine throughput parameters of the two smaller flow passages. The sum of the turbine throughput parameters of these flow passages can be used as a combined value to determine the asymmetry factor, formed from the ratio of the common value for the two smaller flow passages to the value for the larger flow passage. In this embodiment, both smaller flow passages are expediently connected to the exhaust-gas recirculation device and are supplied by a common exhaust line.
In the case where a variable turbine geometry is used, the turbine throughput parameter for the flow passage equipped with variable turbine geometry is determined in the region of the choke line of the exhaust turbine in the position of maximum opening of the turbine geometry.
It may be advantageous to provide a blow-off device as well, which is designed as a bypass to the exhaust turbine and via which an adjustable mass flow of exhaust gas can be guided around the turbine to avoid a critical excess pressure. The blow-off device can likewise be assigned a turbine throughput parameter, which is added to the sum of the throughput parameters for each flow passage in order to establish the absolute size of the exhaust turbocharger to be used. However, the throughput parameter for the blow-off is preferably not included in the determination of the asymmetry factor, i.e. in the determination of the ratio of the throughput parameters through the flow passages. The blow-off device is, in particular, assigned to the smaller flow passage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.